Tuesday, April 29, 2008

Compiler Optimization Can cause problem

Last week, I was created a presentation on Multi-threading in Java. Though this fact, I have covered in presentation but still wanted to blog on same. In multi-threading world, compiler optimization can cause serious problems. Just check my small code:

Check out the reason in bold. Now if compiler optimize the code and remove the part of if(flag), thinking of that flag value will always be false. Then we have a situation here(FBI style of speaking :-D), because other thread can change its value and can make the flag value true. Just run this code 5-6 may be 10 times, you will be able to see the SOP statement "This can be reached". Just for the shake of getting that I have added sleep statement. Here what I got on my 3rd run of the code :)

Value of flag:falseThis can be reachedValue of flag:true

Handling such type of situation is not difficult, specification says to add a word volatile before the variable flag which will tell the compiler not to optimize its code just by seeing some initial value or declaration.